HAFNIUM ISOTOPIC MAPPING OF THE CONTINENTAL LITHOSPHERES OF EASTERN NORTH AMERICA

The isotopic evolution of lithospheric and sub-lithospheric reservoirs is recorded in volcanic rocks associated with extensional/plume tectonics. We utilize basaltic rocks associated with two major supercontinent breakup events, e.g. Rodinia and Pangea, as well as those associated with the formation of the North America Mid-Continent Rift to examine lithosphere evolution over a b.y. of geologic time. The Hf isotope system is used to probe the mantle sources of: the Mid-Continent Rift System (~1100 Ma), Catoctin Volcanic Province (CVP; ~570 Ma), Ouchita Rift Basalts (~550 Ma), Mesozoic Appalachian Tholeiites (MAT; 200 Ma) and the mildly alkaline basalts from Mole Hill in Virginia (~48 Ma). The Mid-Continent Rift System (εHf_i -1.4 to 1.6) and Mesozoic Appalachian Tholeiites (εHf_i 1.3 to 1.5) are similar in εHf_i, and support previous suggestions of the magmas representing melts of the subcontinental lithospheric mantle (SCLM). Although these two volcanic regions are separated by over 700 miles, the similarity in the time corrected isotopic deviation from a chondritic reservoir suggests the possibility of a common lithospheric source under large regions of eastern North America. Volcanism associated with the breakup of Rodinia (e.g. CVP), including development of failed rifts (Ouchita) show measurable differences in εHf_i. The CVP with values of εHf_i from 3.7 to 6.4 and Ouchita (εHf_i 7.9 to 11.1) suggest that the magmas associated with the CVP were derived from a relatively non-radiogenic source. La/Nb ratios of samples from the CVP and the Ouchita rift coupled with εHf_i suggest a complex mixture of several sources, such as depleted mantle, plume (OIB-like source) and SCLM. The data from the CVP are best interpreted as mixtures of plume/lithosphere, while the Ouchita samples may record a depleted mantle and plume interaction. The youngest samples from Mole Hill, VA are dominantly radiogenic (εHf_i 10.3 to 12.2) and are most likely derived from a depleted MORB mantle. The application of Hf isotopes, in conjunction with Nd and Pb isotopes, reveals the complexity of the lithosphere in eastern North America. Our ongoing studies will provide a more comprehensive understanding of lithospheric geochemical signatures associated with ancient magmatic episodes.